BepiColombo: MEFISTO: Mercury Electric Field In-Situ TOol on the Mercury Magnetospheric Orbiter

The BepiColombo orbiters to Mercury, the ESA Mercury Planetary Orbiter (MPO) and the JAXA Mercury Magnetospheric Orbiter (MMO) makes observations of many sides of the planet possible; the interior and the surface of Mercury, the planet's magnetic field an d magnetosphere and the interaction between the solar wind and the magnetosphere. The terrestrial planets with intrinsic magnetic field are the Earth and Mercury only. Detailed observation of the Mercury magnetic field and magnetosphere enables us to do t he first comparative studies with the Earth. This will be a big leap in the studies of planetary magnetospheres. Mercury's magnetosphere is quite different from that of the Earth making it interesting to study in its own right. Yet they are similar enough to allow interesting comparative studies, highly likely to provide new knowledge and improved understanding of planetary magnetospheres. The electric field governs the plasma transport in a magnetosphere. The electric field also plays an important role i n the interaction of the magnetosphere and the underlying ionosphere, in the case of the Earth, or the underlying planetary surface or its immediate environment, in the case of Mercury. Finally, the electric field is an important parameter for the energy transport between the different regions. Understanding of the special features of a different magnetosphere like that of Mercury, will also lead to a deeper understanding of the terrestrial and astronomical magnetospheres. There are also many familiar, Ea rth-like, phenomena which we will learn more about from studying them in a magnetosphere with scale sizes that are vastly different form those of our own, terrestrial ones, and subject to different boundary conditions both at the lower, planetary border, and at its upper, interplanetary border.